A variety of processes are known for synthesizing cimetidine (VI) ##STR1## a compound known to have H.sub.2 -receptor antagonist activity and be of value as a drug for the treatment of gastric and duodenal ulcers.
These processes are disclosed in Japanese Published and Unexamined Patent Applications Kokai Nos. 50-32174, 51-54561, 51-125074 and 54-59275 and Japanese Patent Publication No. 56-1309. These processes, however, are not free from commercial problems; some of them require chromatographic purification of the reaction product and others require use of cysteamine which is an expensive reagent. There also is known a process in which the sodium salt of 4-methyl-5-mercaptomethylimidazole is reacted with N-cyano-N'-methyl-N"-(2-bromethyl)guanidine (Japanese Published and Unexamined Patent Application No. 54-130566) but since the starting material imidazole compound tends to be oxidized under alkaline conditions, the yield of the product is inevitably low. The purpose of the present invention is to overcome these disadvantages.